FIG. 1 depicts a conventional tunneling magnetoresistive (TMR) element 10 having a crystalline MgO tunneling barrier 24. For simplicity, FIG. 1 is not drawn to scale. In addition, although not shown in FIG. 1, the crystalline MgO tunneling barrier 24 could include a thin Mg or Pt layer between the bottom electrode 16 and the MgO in the tunneling barrier 24. The conventional TMR element 10 resides on a substrate 2 on which seed layer(s) 12 have been formed. The conventional TMR element 10 includes a conventional bottom electrode, or pinned layer 16. The conventional bottom electrode 16 shown is a synthetic antiferromagnet that includes magnetic layers 18 and 22 separated by a thin nonmagnetic, conductive spacer layer 20 that might include Ru. The magnetizations (not shown) of the pinned layer 18 and the reference layer 22 are substantially fixed, or pinned. The conventional TMR element 10 also includes a conventional free layer 26, or top electrode 26. The conventional top electrode 26 typically has a magnetization (not shown) that may move, or switch. The conventional reference layer 22, pinned layer 20, and top electrode 26 are typically CoFeB layers. The conventional TMR element 10 may also include a conventional pinning layer 14, such as an antiferromagnetic (AFM) layer 14, that is used to fix the magnetization of the bottom electrode 16. Thus magnetization of the conventional pinned layer 16 is fixed, or pinned, in a particular direction, typically by an exchange-bias interaction with the AFM layer 14. However, the magnetization 21 of the free layer 20 may move, or switch, in response to an external field. Also shown is a conventional capping layer 28, which may be formed of materials such as Ta.
The conventional TMR element 10 is desired to have certain properties, which use of the crystalline MgO tunneling barrier 24 and CoFeB electrodes 16 and 26 may enable. The conventional top electrode 26 is desired to be soft, having a coercivity of not more than five Oersted. A low magnetostriction of λs of not more than approximately 1.0×10−6 (or not less than approximately −1.0×10−6) is also desired. In addition, a low interlayer exchange coupling, Hin of not more than fifty Oersted is desired to help ensure that the magnetization of the conventional top electrode 26 is free to respond to an external field. Thus, CoFeB may be used for the reference layer 22 and the top electrode 26. It is also noted that the top surface of the reference layer 22 is desired to be smooth to improve growth of the MgO tunneling barrier 24. The amorphous structure of CoFeB aids in producing such a surface for the reference layer 22. The conventional MgO tunneling barrier 24 may aid in providing other desired properties for the conventional TMR element 10 if the MgO has the desired crystallographic structure. For example, the MgO should have a near perfect [100] texture and a large grain size. As used herein, a specific texture indicates that the layer has a dominant orientation. Thus, the conventional barrier layer 24 of MgO having a [100] texture means that the conventional MgO barrier layer 24 has a dominant [100] orientation. Use of such a conventional MgO barrier layer 24 may result in a low Ra of not more than 3 Ω/μm2 or a high Q-factor ((ΔR/R)/Ra high) for the conventional TMR element 10. In order to obtain such properties for the conventional TMR element 10, therefore, the crystalline MgO tunneling barrier 24 and CoFeB electrodes 16 and 26 may be used.
Although the conventional TMR element 10 may have the desired properties, issues with the conventional MgO tunneling barrier 24 may adversely affect the properties of the conventional TMR element 10. The conventional MgO tunneling barrier 24 should be substantially impurity-free and have the desired crystallographic structure for the above-identified properties. However, during fabrication of devices including the conventional TMR element 10, elevated temperature anneals may be performed after the conventional MgO tunneling barrier 24 is formed. As a result, boron may diffuse from one or more of the electrodes 16 and 26 to the MgO tunneling barrier layer 24. A conventional TMR device in which boron has diffused into the MgO tunneling barrier is subject to a low Q-factor at low Ra. Consequently, performance of the conventional TMR element 10 is adversely affected.
Accordingly, what is needed is a system and method for improving the performance of TMR elements utilizing MgO tunneling barrier layers.